I went to Steve Miller at DTE a week ago, and sat on the hill. It rained hard for maybe 30 minutes. No stoppage of show. I had brought a 0.7 mil 12'x20' visqueen painters dropcloth in my pocket. We made a dry shelter by sitting on it and wrapping it up behind us and over our heads. Worked great, and we stayed dry. I suggest bringing one for the show. They cost
I once saw Pink Floyd at Clevland Municipal Stadium in a huge thunderstorm. It poured for most of the show, with copious lightning, and they didn't stop that one, either.

Generally the promoter will do anything to keep the show going so refunds are not an issue and bands will usually do anything they can to continue because they want to get paid. As long as the stage is mostly dry and there aren't major issues like wind or lightning, usually the show goes on.

Oh, it was. In that case, I think it added to the show. I drove by Comerica Park during the show, and I think you guys got dumped on pretty hard for a little while, didn't you? But then got to watch lightning for the rest of the night while staying dry. There was a giant rainbow and a beautiful sunset over Detroit last night.

update:
got through Ty Stone, and got 4 songs out of Sammy Hagar before it blew in. Sammy kicked ***, he's in great voice, so it's too bad the storm cut him short. They cleared us all out of the field and uncovered stands until the storm blew by. 45 minutes later, Kid Rock took the stage and tore the place down!